It has been observed that direct incorporation of corrosion-inhibiting functional molecules and/or nano-objects in large quantities in a matrix reduces both the mechanical and barrier properties of the host matrix, leading to insufficient anticorrosion protection for the coating. What is more, this direct incorporation may be incompatible with the chemical nature and inhibition method for certain classes of corrosion inhibitors. Indeed, most organic corrosion inhibitors are chelating/complexing agents for metal ions and inorganic corrosion inhibitors are often salts of metal ions. From this observation, it is clear that mixing these two classes of inhibitors may, in some cases, lead to partial neutralization of the corrosion inhibitors (inorganic corrosion inhibitors complexed by organic corrosion inhibitors). To at least partially remedy the negative effects of direct incorporation of corrosion inhibitors in the coating, the strategy of incorporating corrosion inhibitors in nanomaterials has been studied in recent years. Different types of particles having the function of serving as nano-reservoirs have been tested, such as non-porous SiO2 nanoparticles coated with polyelectrolytes (alternating multiple layers of polymers with opposite charges obtained by layer-by-layer deposit), boehmite nanoparticles, halloysite nanotubes, layered double hydroxides, hydroxyapatite microparticles, or mesoporous nanoparticles of silica coated with polyelectrolytes. However, the use of such particles presents three major drawbacks: the quantity of corrosion inhibitors present in the particles, and therefore in the coating, is limited, the release kinetics for the inhibitors incorporated into the particles are slow and the synthesis of these systems active on corrosion is long and involves a large number of steps, which makes these particles hard to industrialize.
It has been discovered that incorporating corrosion inhibitors indirectly into the coatings, in mesostructured particles, can significantly increase the level of corrosion inhibitors loaded in a coating without altering its macroscopic or microscopic properties.
Two main mesostructuration processes for materials are known. The first mechanism, called liquid crystal templating, involves the prior existence of a liquid-crystal phase before the condensation of inorganic species. The formation of the material then results from the diffusion of inorganic precursors in the inter-micellar spaces in the organic mesophase. The second mechanism relies on the phenomenon of cooperative self-assembly, in which surfactant molecules and inorganic species combine in a first step to form an intermediate hybrid mesophase. By combining sol-gel chemistry (hydrolysis-condensation of inorganic and/or hybrid organic-inorganic precursors) and a liquid-crystal phase (pre- or post-formed), it is therefore possible to develop a material that has periodic-phase nanosegregation that can lead, depending on the mesophases obtained, to the existence within the particles of at least one three-dimensional network that may be inorganic or hybrid organic-inorganic, and where the other phases may be purely organic, hybrid organic-inorganic or inorganic. Materials having such phase nanosegregation are defined as being mesostructured materials.
The use properties for such materials are intimately connected to the porosity release by elimination of the surfactant phase, which is generally obtained from chemical extraction methods or by high-temperature heat treatments (500° C.). Mesostructured materials whose porosity has been released are defined as periodically organized mesoporous materials.
Mesostructured materials synthesized in powder form have most commonly been obtained from methods of synthesis by precipitation. Generally, these need an autoclave curing step that is often long (from 12 to 24 h) and is incompatible with continuous production. What is more, the stoichiometry of the initial solution and the final material may differ if a portion of the reagents is found in the supernatant. Finally, with this technique it is difficult to obtain elemental particles that have a regular shape and size.
An alternative to methods of synthesis by precipitation, less frequently used and relying on the phenomenon of cooperative self-assembly, involves the evaporation of solvents from dilute solutions of reagents. The principle of this method, commonly called evaporation-induced self-assembly (EISA), consists in causing self-assembly of surfactants in the liquid-crystal phase and the condensation of inorganic and/or organic-inorganic hybrid precursors present around micellar aggregates, when the solvents evaporate. From this synthetic strategy in the last decade mesostructured materials have been developed in the form of films, micro-units, membranes, fibers and submicron particles using industrializable, or even industrial, shaping processes.
The synthesis of materials using EISA first involves the development of an aqueous or dilute water-alcohol solution containing the inorganic and/or hybrid precursors, the catalysts and/or inhibitors for hydrolysis-condensation reactions (respectively, in the case of silicic and transition metal oxide precursors), surfactants and the functional molecules and/or nano-objects. This solution may then be deposited on to a substrate either by dip-, spin- or spray-coating to form a film, or sprayed in spherical droplets to obtain spherical particles via the aerosol method. The material then undergoes an evaporation phase at moderate temperatures (less than 250° C.) to allow self-assembly of the surfactants and partial condensation of the inorganic and/or hybrid precursors around micellar aggregates. The material obtained may then undergo a post-treatment that aims to consolidate the inorganic or hybrid phase.
In comparison with the precipitation method, the evaporation method presents several advantages, such as better control of reagent hydrolysis-condensation, controlling stoichiometry for particles equal to that of the stoichiometry in non-volatile species from the initial solution, obtaining more monodispersed spherical particles, synthesizing powder continuously, controlling particle size and mesophase by adjusting the physical and chemical parameters of the solution and the parameters of the aerosol method, the possibility of working with heterogeneous solutions containing (nano)particles for example, or even the possibility of simply making core-shell particles by using double concentric nozzles, the possibility of obtaining much higher loading levels of functional molecules and/or nano-objects and the possibility of working with chemically incompatible compounds through the use of double concentric nozzles.
The Sanchez et al. review (Design, synthesis, and properties of inorganic and hybrid thin films having periodically organized nanoporosity, 2008, Chemistry Materials) presents in a detailed manner the different preparation and shaping routes for these materials in the form of thin films, underlining the advantages and drawbacks of each, and focusing on the influence of many synthetic parameters and the mechanisms at play. In the same way, the scientific publication Boissiere et al. (Aerosol Route to Functional Nanostructured Inorganic and Hybrid Porous Materials, 2011, Advanced Materials) presents the different strategies that lead to obtaining mesostructured and/or mesoporous particles by combining sol-gel chemistry, self-assembly of surfactants and the aerosol spray method.
The solution described in the article by Jiang et al. (Controlled Release from Core-Shell Nanoporous Silica Particles for Corrosion Inhibition of Aluminum Alloys, 2011, Journal of Nanomaterials) to obtain particles loaded with corrosion inhibitors uses the EISA aerosol route and incorporation of corrosion inhibitors was achieved in a single step at the same time as the formation of mesostructured particles. However, the authors conducted a calcination step at 500° C. for five hours, which caused removal of the surfactants, degradation of the corrosion inhibitors and crystallization of the corrosion inhibitors. Such a step includes the drawback of not being compatible with either the incorporation of organic inhibitors or with the incorporation of inorganic inhibitors in molecular and/or non-oxide forms.
Another solution is presented in the articles by Shchukin et al. (Surface-Modified Mesoporous SiO2 Containers for Corrosion Protection, 2009, Advanced Functional Materials; Mesoporous Silica Nanoparticles for Active Corrosion Protection, 2011, ACS Nano; Influence of Embedded Nanocontainers on the Efficiency of Active Anticorrosive Coatings for Aluminum Alloys Part I: Influence of Nanocontainer Concentration, 2012, ACS Applied Materials & Interfaces; Influence of Embedded Nanocontainers on the Efficiency of Active Anticorrosive Coatings for Aluminum Alloys Part II: Influence of Nanocontainer Position, 2013, ACS Applied Materials & Interfaces). However, the particles described in these articles are not mesostructured but are mesoporous. What is more, the development of particles loaded with corrosion inhibitors was achieved after a long (48 hours), multi-step synthesis process and post-treatments, a process that is difficult to see as compatible with industrial use. The particles described in these articles have been obtained by the classic precipitation pathway and not by the spraying of a solution. Finally, the mesoporous particles were loaded with corrosion inhibitors according to an iterative process of absorption in solution, which is a long, restrictive process that limits the load level and causes a large quantity of effluents that have to be reprocessed.
Mesostructured matrices have three distinct regions at the nanometric scale: (a) the inorganic and/or hybrid network, (b) the aqueous interface composed of M-OH/M-O− groups (where M is a metal or silicon), H2O, and the polar surfactant heads, and (c) the hydrophobic core of micelle aggregates. Solubilization of functional molecules in mesostructured matrix is mainly governed by the following principle: “like dissolves like.” This principle implies that a polar molecule is likely to be located either in the inorganic and/or hybrid network (as long as the hybrid portion is itself polar) or at the aqueous interface, and that a non-polar molecule is likely solubilized in the hydrophobic core of the micelle phase.
However, this principle is statistical and does not actually reflect the diffusion of molecules within mesostructured materials. Indeed, molecules can migrate from the hydrophobic portion of the micelles to the aqueous interface (and the reverse) as a function of the thermal stirring in the medium or chemical reactions (protonation-deprotonation). What is more, the diffusion of molecules is not only limited to the nanometric scale between these three regions but may occur in the mesostructure over a larger distance in a relatively short time (several tens of microns in a few minutes). Many parameters influence the diffusion of molecules in mesostructured materials such as size, load, the hydrophilic/hydrophobic balance of molecules, intra- or intermolecular interactions, the type of mesostructure (lamellar, 2D-hexagonal, vermicular, cubic), the pore size, whether surfactants are present or not, the nature of the surfactants (cationic, anionic or nonionic), the quantity of M-OH/M-O− at the interface, the quantity of water in the material, the nature of the surface of the pores (inorganic or organically modified), the interconnection between pores, how complex the network is, etc.
From these studies, the general behavior of how a molecule diffuses in a mesostructured material may be formulated: “the lower the interactions between the functional matrix and the molecule, the faster and more easily a molecule diffuses.” Indeed, the diffusion of a molecule in a mesostructured matrix is not linear but consists in a succession of adsorption-desorption phases and diffusion phases. As an example, molecules solubilized in the surfactant phase (which can be assimilated in the solvent) diffuse faster than molecules interacting with adsorption sites on the surface of the inorganic portion. It is therefore easier for molecules to diffuse if the surface of the inorganic portion is passivated by inert functions on diffusing molecules.
A coating comprising mesostructured particles may be shaped by several different deposit techniques. The most well-known techniques are dip-coating, spin-coating, coil-coating and roll-coating, capillary-coating, doctor blade and spray-coating. Among all these techniques, the best that delivers an even coating from a weakly stable suspension, a coating in which the particles will be distributed statistically on the coating's entire thickness, is spray-coating, since homogeneity is an important parameter in terms of reproducibility, mechanical properties and anticorrosion activity. Spray coating also allows the deposit of coating on large parts and complex shapes.